Satellite radionavigation uses certain propagation properties of radioelectric waves to determine a position and a speed of a moving object on the basis of signals emitted from satellites. The durations of propagation of the radioelectric waves emitted by the satellites make it possible to determine pseudo-distances between the moving object and the satellites. Through resolution akin to triangulation, it is possible to deduce the position of the moving object. The signals emitted by satellites not being very powerful, they are easily jammed. Jamming constitutes one of the main threats in regard to the availability and continuity of satellite radionavigation service. Numerous antijamming solutions exist or are under development. In the majority of these solutions, the antijamming system implemented comprises a radiofrequency chain for selection, amplification and digitization of the signal over a band of frequencies. The signal is notably quantized in amplitude on a given scale and with a given precision. The optimal use of the quantization scale naturally depends on the power of the received signal and the signal amplification gain. Consequently, the amplification gain is generally regulated as a function of the power of the signal received by a so-called automatic gain correction circuit.
The power of the useful signal, that is to say of the signal originating from a satellite, is situated about 30 dB below the thermal noise, whereas the power of the jamming or interference signals is in general considerably greater than the power of the thermal noise. In the absence of jamming signals, the amplification of the signals is tailored for optimal quantization of the thermal noise. Quantization on a low number of bits, for example between one and three, can suffice to demodulate the signal and exploit it. On the other hand, in the presence of jamming signals, the amplification of the signals is tailored for optimal quantization of the jamming signals, thus leading to the loss of the useful signal information and thermal noise information if the quantization is carried out on a low number of bits. In order to be able to quantize the useful signal and the thermal noise, the quantization must therefore be carried out on a sufficient number of bits, of the order of 6 or 7 bits minimum. A problem nonetheless arises as regards the level of the regulating setpoint to be applied. If the setpoint level is close to the bottom of the scale of the quantizer, the jamming signals of short duration (not affecting regulation) are quantized over the whole of the scale and can therefore be suppressed by linear mathematical processing. On the other hand, the continuous jamming signals (taken into account in regulation) are quantized solely around the low values of the scale. It is then difficult to characterize them and to filter them. Furthermore, the useful signal risks being lost. Conversely, if the setpoint level is close to the top of the scale, the useful signal and the continuous jamming signals are correctly quantized. However, the jamming signals of short duration are saturated and therefore poorly processed. Moreover, in the absence of jamming, the gain in amplification is continually very high since it must raise the thermal noise level, which is of the order of −100 dBm at the input of the antenna of the radionavigation system, to the top of the quantization scale. The gain in amplification can thus attain 100 dB. Such an amplification is all the more difficult to carry out as the radiofrequency chain generally has a small shape factor and uses a unique frequency transposition. A compromise is to apply a setpoint level in the middle of the scale. However, such a setpoint level does not allow optimal quantization of the useful signal, the jamming signals of short duration, and the continuous jamming signals.